Magnetometers are often used in the wellbore as devices that aid in surveying. For example, a down-hole tool may include magnetometers to measure the position of the tool versus the Earth magnetic vector and to determine the azimuth of the tool. Magnetic measurements such as these may be used to obtain wellbore azimuth as well as the magnetic tool face of the tool in the well. The wellbore inclination and azimuth taken together with other survey data such as measured depth, make it possible to determine the wellbore trajectory, thereby aiding the driller in implementation of the drilling plan.
Measurement while drilling (MWD) tools allow data to be collected while drilling. The collected data may then be transmitted to the surface, e.g., using mud pulse or electromagnetic (Emag) telemetry. The ability of MWD tools to collect a wealth of data underlies the current success of modern directional drilling techniques. For example, measurement of the Earth's magnetic flux while downhole allows for the determination of magnetic tool face (the angular orientation of the tool in the well) thereby allowing the operator to set the angular orientation of a device in a wellbore, e.g., the bend of a steering motor or the orientation of a whipstock. Furthermore, magnetic tool face can be measured in a dynamic fashion during rotation of the drill collar. This type of dynamic measurement technique is commonly employed in rotary steerable systems (RSS). Dynamic data logging is also employed with logging while drilling (LWD) tools. For example, in dynamic data LWD devices, magnetic tool face may be continuously determined while the collar rotates to allow the proper mapping of logged data versus angular position in the wellbore. Furthermore, a measurement of the tool-face versus time allows for the calculation of the rotational velocity and acceleration of the tool by way of the first and second derivatives, respectively, of the tool-face-versus-time data.
Down-hole magnetometers may be very sensitive to external perturbations of the Earth's magnetic field. Thus, theses magnetometers should be deployed within non-magnetic tubulars or drill collars. Furthermore, extra lengths of non-magnetic drill collar should be used in bottom hole assemblies (BHAs) that employ MWD tools having magnetometers to ensure that the magnetometer measurements are not systematically affected by any surrounding magnetic material. For similar reasons, in directional drilling operations, kick-off from vertical in wells in close proximity to many other cased wells is preferably done using non-magnetic based navigation systems. For similar reasons, the orientation of a whipstock inside casing is preferably not performed with magnetometers.